a) Field of the Invention
The invention is directed to an illumination condenser for a particle optics projection system.
b) Description of the Related Art
In semiconductor fabrication, it is often necessary to illuminate an object (mask or multi-beam modulator) telecentrically by means of a condenser. In so doing, the partial crossover belonging to an off-axis point of the mask is usually directed away from the axis due to the spherical aberration of the condenser, which results in additional errors (inclined beam incidence in target, astigmatism and/or distortion). The off-axis crossover causes additional aberrations in the particle optics projection device because the beams pass the lens fields at a greater distance from the axis. It is essentially the spherical aberration of the illumination condenser that causes the partial crossover to be off-axis. The effect of the spherical aberration of the illumination condenser is proportional to the third power of the operative aperture in the crossover. In large-field particle optics projection systems, this aperture is very large, typically 100 mrad. It can be reduced in existing illumination condensers only by a very large distance between the particle source and the mask or multi-aperture multi-beam modulator. However, this results in an excessive structural height of the particle beam projection system.
U.S. Pat. No. 5,742,062 describes an electrostatic lens which comprises a plurality of annular electrodes to which different potentials are applied. This lens is suited to enable a large-field parallel illumination of an object (mask) with charged particles. It is possible to minimize lens aberrations through suitable selection of the electrode potentials. It is disadvantageous that elaborate technical resources are required to provide a plurality of different potentials in a highly consistent manner over time. In case of high beam energy (50 . . . 200 keV), long insulation distances are required, the vacuum feedthroughs are correspondingly large, and therefore the required vacuum volumes for this arrangement are also large. This leads to the disadvantage that the shielding of the particle beam from magnetic interference fields is very extensive because suitable shielding cylinders, e.g., made of Mu metal, require a large radius and thick walls. Further, the outgassing of the large surface of the electrodes and insulators is disadvantageous for the desired low final pressure in the chamber and for a suitably low contamination rate.
Rose's paper on the magnetic-electrostatic multielectrode lens (D. Preikszas, H. Rose, Optik 100 (1995), 179) discloses a combined magnetic-electrostatic lens comprising a quantity of identical elements (apertures, ring currents) which are controlled in such a way that aberrations are suppressed to a great extent.
The prior art is characterized in that the spherical aberration of the illumination condenser in particle beam projection devices is reduced, if at all, only over the large radial extension of the condenser. This substantially limits the possible size of the mask and/or of the multi-aperture beam modulator. There have only been attempts with electrostatic condenser lenses having very many electrodes (multi-aperture condenser) to achieve a shape of the electrostatic field of the condenser such that the spherical aberration is extensively suppressed. However, an electrostatic multi-aperture condenser of this kind requires a large quantity of high-voltage supplies for the individual electrodes and is therefore only usable to a limited extent.